The present invention relates generally to color imaging assemblies which employ multilayered dichroic composites for generating spatially separated, color component images of an object on an image plane and also to color combiners which employ multilayered dichroic composites for combining separate beams of light of different spectral ranges into a single combined beam. The invention relates particularly to path length compensators which are used in association with dichroic composites.
The phrase "beam of light" is sometimes narrowly defined to mean a bundle of parallel light rays such as those generated by a collimated light source. The phrase "beam of light" may also be more broadly defined to mean any narrow shaft of light having light rays traveling in the same general direction. Used in this broader sense, the light which emanates from an object and passes through the aperture of an imaging lens as well as the converging cone of light which emerges from the lens and which is focused on an image plane may be collectively referred to as a "beam of light". When the phrase "beam of light" is used herein, it is to be understood that this broader meaning is intended.
Vincent, U.S. Pat. No. 4,709,144 and Vincent et al., U.S. Pat. No. 4,870,268, which are hereby specifically incorporated by reference for all that is disclosed therein, describe a number of different dichroic composites which are used in beam splitter assemblies and beam combiner assemblies. An optical scanner which employs a beam splitter is described in U.S. patent application Ser. No. 383,463 filed July 20, 1989, for OPTICAL SCANNER of David Wayne Boyd, now U.S. Pat. No. 4,926,041 which is hereby specifically incorporated by reference for all that it discloses.
Problems associated with differences in optical path lengths in separated component beams arise when dichroic composites are used in beam splitters and/or beam combiners. Certain prior art beam splitter assemblies and associated techniques for resolving optical path length problems which are disclosed in U.S. Pat. Nos. 4,709,144 and 4,870,268 will now be briefly described.
FIG. 1 is a schematic side elevation view of a line-focus-type color imaging assembly comprising a line object 1 which originates a polychromatic light beam 4 which passes through an imaging lens 6 which is adapted to focus a line image of the line object on an image plane II located at a fixed optical path length distance from the imaging lens 6. The light beam 4 impinges upon a dichromatic beam splitter 56 which splits the polychromatic light beam 4 into spectrally and spatially separated color component beams 8, 9, 10 which provide focused color component images of the line object on a monolithic photosensor unit 11, FIGS. 1 and 2, positioned at the image plane II.
FIG. 1 illustrates the manner in which two optically flat transparent optical support media 60 and 62 can be attached to provide three substantially equally spaced dichroic coatings to produce three substantially parallel optical component beams 8, 9, 10 that are both spatially and spectrally separated. The optical separator 56 consists of precisely ground and polished glass plates 60 and 62 coated on one or both faces with dichroic coatings 50, 52 and 54. At each dichroic coating 50, 52 and 54, incident light is either reflected or transmitted according to wavelength with negligible absorption loss. The composition of the dichroic coatings 50, 52 and 54 can be designed for accurate bandpass filtration.
The plate 60, shown in FIG. 1, is designed such that incident light striking dichroic coating 50 at 45.degree. reflects blue light (approximately 400-500 nm) while transmitting red light and green light.
Plate 62, shown in FIG. 1, is coated on both faces with dichroic coatings 52 and 54 such that an incident polychromatic light beam 4 striking a first dichroic coating 52 at nominally 45.degree. reflects the red spectral band (e.g., 600-700 nm) while transmitting the green band. The green light striking a second dichroic coating 54 and having an optical axis oriented nominally 45.degree. from the dichroic coating is reflected. The reflected green light is caused to pass back through the glass plate 62 and through the other dichroic coatings 52 and 50 at a 45.degree. angle. As shown in FIG. 1, each of the color components 8, 9 and 10 of the incident light are reflected at 90 to incoming beam 4. The reflected red and green components 9 and 8 are parallel and separated from each other by a distance determined by the glass plate 62 and dichroic coating thickness 52, the index of refraction of plate 62, and the angle of incidence. Similarly, the blue and red components 10 and 9 are separated by a distance determined by the thickness of the glass plate 60, dichroic coating 50, the index of refraction of the plate 60 and the angle of incidence.
A mirror coating could be substituted for the third dichroic coating 54, since only the third remaining color component reaches that coating interface.
A suitable photosensor unit 11 for use with optical separator 56 is shown in FIG. 2. Photosensor 11 may be a single chip, single package solid state device having three linear photosensor arrays, 12, 13 and 14, precisely aligned and spaced to coincide with the focused line images formed by beams 8, 9 and 10, respectively, shown in FIG. 1.
As illustrated in FIG. 1, light in each of the color component beams 8, 9, 10 travels a different optical path length through the beam splitter 56. As a result in the differences in component beam light path length through beam splitter 56, photosensor unit 11 is skewed at an angle theta relative to a component beam normal plane such that the total optical path lengths of each of the different color components, as measured from lens 6 to the photosensor unit 11, are equal. Angle theta and the distance "D" between linear photosensor arrays 13, 14 are functions of glass plate and dichroic layer thickness X and index of refraction.
FIG. 3 shows a beam splitter/photosensor arrangement which enables photosensor 11 to be positioned perpendicular to the optical axes of the color-separated beams. In this arrangement, the path-lengths-through-glass of the color-separated beams are made equal by the reciprocal arrangement of trichromatic beam splitters 56 and 58.
As shown in FIG. 3, the incident light beam 4 is aligned to impinge the hypotenuse face 32 of right angle prism 51 at a normal angle and transmit therein to a first base side 30 of the prism 51 which the light beam impinges at 45.degree.. The composite beam splitter 56 of FIG. 1 is attached thereto. A trichromatic separation of the red, green and blue spectral components of the incident light beam occurs as previously described. The three reflected component beams re-enter the prism 51 and are directed toward the second base side 34 of prism 51, each separated beam impinging the second base side 34 at 45.degree. incidence. A second composite beam splitter 58 is attached to the second base side 34 of prism 51. The plates 60 and 62 and the dichroic coatings 50, 52 and 54 in beam splitters 56 and 58 are identical. However, the orientation of the composite beam splitters 56 and 58, and the multilayer dielectric coatings 50, 52 and 54 on each base side 30 and 34 of the prism 51 are reversed so that the path lengths of each component color beam entering and exiting the trichromatic prism beam splitter 59 are identical. That is, a component color beam, such as blue, reflects off the dichroic coating 50 on plate 60 located on base side 30. Next, the blue component reflects off the dichroic coating 50 on plate 60 located adjacent to base side 34. In a like manner, a red component color beam goes from middle filter 52 on base side 30 to middle filter 52 on base side 34, and the green component reflects off a backside filter 54 to a front side filter 54. Reflected beams from the trichromatic beam splitter 58 adjacent to base side 34 are directed out of prism 51. The beams are perpendicular to the hypotenuse side 32 and parallel to the incident light beam. The thickness of the beam splitter glass plates, 60 and 62, and the dichroic coatings, 50, 52 and 54, determine the separation of the reflected beams. Thus, the dual trichromatic beam splitter 59 provides an equal path length through the glass for all color components. Also, the light enters and leaves the prism at a normal angle of incidence.
Referring to FIG. 4, a fluorescent light source 22 illuminates the surface of an original document 21. A beam of imaging light from the original document is projected onto a beam splitter assembly, consisting of dichroic beam splitters 16 and 17, by lens 6. Beam splitters 16 and 17 are flat glass plates coated on one side with dichroic coatings 50 and 52, respectively. Beam splitter 16 is designed to reflect blue light while transmitting red and green spectral bands. The blue light is reflected to a first CCD linear-array photosensor 18, with beam splitter 16 tilted at 45.degree. to the incident light beam 4. Beam splitter 17 reflects red light to a second CCD photodiode array sensor 20. The green line image passing through both beam splitter plates is captured by the third CCD photodiode array sensor 19. Beam splitter plate 17 is also aligned at 45.degree.. to the incident light beam 4, as shown. In this arrangement in which each linear photosensor array 18, 19, 20 is provided on a separate photosensor unit differences in optical path lengths of the color component beams through beam splitters 16, 17 are compensated by individually adjusting the positions of the different photosensor units.
Thus, it is known in the prior art to adjust for component beam path length differences through a beam splitter by skewing a unitary plane, photosensor array relative a component beam normal plane. A problem with this solution is that skewing the photosensor array reduces the illumination of the photosensor array by an amount proportional to the cosine of the angle of skew. Such a skewed sensor configuration also produces astigmatism which reduces the modulated transfer function (MTF) of the image.
It is also known in the prior art to correct for component beam path length differences through a beam splitter by providing a second, complimentary beam splitter configuration. However, such a solution increases the total cost of optical system due to the additional cost of the second beam splitter. The second beam splitter may also compound optical degradation associated with the use of any dichroic beam splitter.
It is also known in the prior art to correct for path length differences through a beam splitter assembly by providing a different photosensor unit for each component beam. However, such an arrangement is not as compact as the other discussed arrangements and is subject to additive, tolerance-related problems of the type experienced in optical systems having multiple optical components which must be precisely positioned and aligned. A further problem is the additional expense associated with using multiple photosensor units.